From: Steven Rostedt (VMware) rostedt@goodmis.org
commit ed65df63a39a3f6ed04f7258de8b6789e5021c18 upstream.
While writing an email explaining the "bit = 0" logic for a discussion on making ftrace_test_recursion_trylock() disable preemption, I discovered a path that makes the "not do the logic if bit is zero" unsafe.
The recursion logic is done in hot paths like the function tracer. Thus, any code executed causes noticeable overhead. Thus, tricks are done to try to limit the amount of code executed. This included the recursion testing logic.
Having recursion testing is important, as there are many paths that can end up in an infinite recursion cycle when tracing every function in the kernel. Thus protection is needed to prevent that from happening.
Because it is OK to recurse due to different running context levels (e.g. an interrupt preempts a trace, and then a trace occurs in the interrupt handler), a set of bits are used to know which context one is in (normal, softirq, irq and NMI). If a recursion occurs in the same level, it is prevented*.
Then there are infrastructure levels of recursion as well. When more than one callback is attached to the same function to trace, it calls a loop function to iterate over all the callbacks. Both the callbacks and the loop function have recursion protection. The callbacks use the "ftrace_test_recursion_trylock()" which has a "function" set of context bits to test, and the loop function calls the internal trace_test_and_set_recursion() directly, with an "internal" set of bits.
If an architecture does not implement all the features supported by ftrace then the callbacks are never called directly, and the loop function is called instead, which will implement the features of ftrace.
Since both the loop function and the callbacks do recursion protection, it was seemed unnecessary to do it in both locations. Thus, a trick was made to have the internal set of recursion bits at a more significant bit location than the function bits. Then, if any of the higher bits were set, the logic of the function bits could be skipped, as any new recursion would first have to go through the loop function.
This is true for architectures that do not support all the ftrace features, because all functions being traced must first go through the loop function before going to the callbacks. But this is not true for architectures that support all the ftrace features. That's because the loop function could be called due to two callbacks attached to the same function, but then a recursion function inside the callback could be called that does not share any other callback, and it will be called directly.
i.e.
traced_function_1: [ more than one callback tracing it ] call loop_func
loop_func: trace_recursion set internal bit call callback
callback: trace_recursion [ skipped because internal bit is set, return 0 ] call traced_function_2
traced_function_2: [ only traced by above callback ] call callback
callback: trace_recursion [ skipped because internal bit is set, return 0 ] call traced_function_2
[ wash, rinse, repeat, BOOM! out of shampoo! ]
Thus, the "bit == 0 skip" trick is not safe, unless the loop function is call for all functions.
Since we want to encourage architectures to implement all ftrace features, having them slow down due to this extra logic may encourage the maintainers to update to the latest ftrace features. And because this logic is only safe for them, remove it completely.
[*] There is on layer of recursion that is allowed, and that is to allow for the transition between interrupt context (normal -> softirq -> irq -> NMI), because a trace may occur before the context update is visible to the trace recursion logic.
Link: https://lore.kernel.org/all/609b565a-ed6e-a1da-f025-166691b5d994@linux.aliba... Link: https://lkml.kernel.org/r/20211018154412.09fcad3c@gandalf.local.home
Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Petr Mladek pmladek@suse.com Cc: Ingo Molnar mingo@redhat.com Cc: "James E.J. Bottomley" James.Bottomley@hansenpartnership.com Cc: Helge Deller deller@gmx.de Cc: Michael Ellerman mpe@ellerman.id.au Cc: Benjamin Herrenschmidt benh@kernel.crashing.org Cc: Paul Mackerras paulus@samba.org Cc: Paul Walmsley paul.walmsley@sifive.com Cc: Palmer Dabbelt palmer@dabbelt.com Cc: Albert Ou aou@eecs.berkeley.edu Cc: Thomas Gleixner tglx@linutronix.de Cc: Borislav Petkov bp@alien8.de Cc: "H. Peter Anvin" hpa@zytor.com Cc: Josh Poimboeuf jpoimboe@redhat.com Cc: Jiri Kosina jikos@kernel.org Cc: Miroslav Benes mbenes@suse.cz Cc: Joe Lawrence joe.lawrence@redhat.com Cc: Colin Ian King colin.king@canonical.com Cc: Masami Hiramatsu mhiramat@kernel.org Cc: "Peter Zijlstra (Intel)" peterz@infradead.org Cc: Nicholas Piggin npiggin@gmail.com Cc: Jisheng Zhang jszhang@kernel.org Cc: =?utf-8?b?546L6LSH?= yun.wang@linux.alibaba.com Cc: Guo Ren guoren@kernel.org Cc: stable@vger.kernel.org Fixes: edc15cafcbfa3 ("tracing: Avoid unnecessary multiple recursion checks") Signed-off-by: Steven Rostedt (VMware) rostedt@goodmis.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- kernel/trace/ftrace.c | 4 +- kernel/trace/trace.h | 64 ++++++++++++----------------------------- kernel/trace/trace_functions.c | 2 - 3 files changed, 23 insertions(+), 47 deletions(-)
--- a/kernel/trace/ftrace.c +++ b/kernel/trace/ftrace.c @@ -6336,7 +6336,7 @@ __ftrace_ops_list_func(unsigned long ip, struct ftrace_ops *op; int bit;
- bit = trace_test_and_set_recursion(TRACE_LIST_START, TRACE_LIST_MAX); + bit = trace_test_and_set_recursion(TRACE_LIST_START); if (bit < 0) return;
@@ -6411,7 +6411,7 @@ static void ftrace_ops_assist_func(unsig { int bit;
- bit = trace_test_and_set_recursion(TRACE_LIST_START, TRACE_LIST_MAX); + bit = trace_test_and_set_recursion(TRACE_LIST_START); if (bit < 0) return;
--- a/kernel/trace/trace.h +++ b/kernel/trace/trace.h @@ -518,23 +518,8 @@ struct tracer { * When function tracing occurs, the following steps are made: * If arch does not support a ftrace feature: * call internal function (uses INTERNAL bits) which calls... - * If callback is registered to the "global" list, the list - * function is called and recursion checks the GLOBAL bits. - * then this function calls... * The function callback, which can use the FTRACE bits to * check for recursion. - * - * Now if the arch does not suppport a feature, and it calls - * the global list function which calls the ftrace callback - * all three of these steps will do a recursion protection. - * There's no reason to do one if the previous caller already - * did. The recursion that we are protecting against will - * go through the same steps again. - * - * To prevent the multiple recursion checks, if a recursion - * bit is set that is higher than the MAX bit of the current - * check, then we know that the check was made by the previous - * caller, and we can skip the current check. */ enum { TRACE_BUFFER_BIT, @@ -547,12 +532,14 @@ enum { TRACE_FTRACE_NMI_BIT, TRACE_FTRACE_IRQ_BIT, TRACE_FTRACE_SIRQ_BIT, + TRACE_FTRACE_TRANSITION_BIT,
- /* INTERNAL_BITs must be greater than FTRACE_BITs */ + /* Internal use recursion bits */ TRACE_INTERNAL_BIT, TRACE_INTERNAL_NMI_BIT, TRACE_INTERNAL_IRQ_BIT, TRACE_INTERNAL_SIRQ_BIT, + TRACE_INTERNAL_TRANSITION_BIT,
TRACE_BRANCH_BIT, /* @@ -592,12 +579,6 @@ enum { * function is called to clear it. */ TRACE_GRAPH_NOTRACE_BIT, - - /* - * When transitioning between context, the preempt_count() may - * not be correct. Allow for a single recursion to cover this case. - */ - TRACE_TRANSITION_BIT, };
#define trace_recursion_set(bit) do { (current)->trace_recursion |= (1<<(bit)); } while (0) @@ -617,12 +598,18 @@ enum { #define TRACE_CONTEXT_BITS 4
#define TRACE_FTRACE_START TRACE_FTRACE_BIT -#define TRACE_FTRACE_MAX ((1 << (TRACE_FTRACE_START + TRACE_CONTEXT_BITS)) - 1)
#define TRACE_LIST_START TRACE_INTERNAL_BIT -#define TRACE_LIST_MAX ((1 << (TRACE_LIST_START + TRACE_CONTEXT_BITS)) - 1)
-#define TRACE_CONTEXT_MASK TRACE_LIST_MAX +#define TRACE_CONTEXT_MASK ((1 << (TRACE_LIST_START + TRACE_CONTEXT_BITS)) - 1) + +enum { + TRACE_CTX_NMI, + TRACE_CTX_IRQ, + TRACE_CTX_SOFTIRQ, + TRACE_CTX_NORMAL, + TRACE_CTX_TRANSITION, +};
static __always_inline int trace_get_context_bit(void) { @@ -630,59 +617,48 @@ static __always_inline int trace_get_con
if (in_interrupt()) { if (in_nmi()) - bit = 0; + bit = TRACE_CTX_NMI;
else if (in_irq()) - bit = 1; + bit = TRACE_CTX_IRQ; else - bit = 2; + bit = TRACE_CTX_SOFTIRQ; } else - bit = 3; + bit = TRACE_CTX_NORMAL;
return bit; }
-static __always_inline int trace_test_and_set_recursion(int start, int max) +static __always_inline int trace_test_and_set_recursion(int start) { unsigned int val = current->trace_recursion; int bit;
- /* A previous recursion check was made */ - if ((val & TRACE_CONTEXT_MASK) > max) - return 0; - bit = trace_get_context_bit() + start; if (unlikely(val & (1 << bit))) { /* * It could be that preempt_count has not been updated during * a switch between contexts. Allow for a single recursion. */ - bit = TRACE_TRANSITION_BIT; + bit = start + TRACE_CTX_TRANSITION; if (trace_recursion_test(bit)) return -1; trace_recursion_set(bit); barrier(); - return bit + 1; + return bit; }
- /* Normal check passed, clear the transition to allow it again */ - trace_recursion_clear(TRACE_TRANSITION_BIT); - val |= 1 << bit; current->trace_recursion = val; barrier();
- return bit + 1; + return bit; }
static __always_inline void trace_clear_recursion(int bit) { unsigned int val = current->trace_recursion;
- if (!bit) - return; - - bit--; bit = 1 << bit; val &= ~bit;
--- a/kernel/trace/trace_functions.c +++ b/kernel/trace/trace_functions.c @@ -138,7 +138,7 @@ function_trace_call(unsigned long ip, un pc = preempt_count(); preempt_disable_notrace();
- bit = trace_test_and_set_recursion(TRACE_FTRACE_START, TRACE_FTRACE_MAX); + bit = trace_test_and_set_recursion(TRACE_FTRACE_START); if (bit < 0) goto out;