From: "Paul E. McKenney" paulmck@kernel.org

commit 96017bf9039763a2e02dcc6adaa18592cd73a39d upstream.

Currently, trc_wait_for_one_reader() atomically increments the trc_n_readers_need_end counter before sending the IPI invoking trc_read_check_handler(). All failure paths out of trc_read_check_handler() and also from the smp_call_function_single() within trc_wait_for_one_reader() must carefully atomically decrement this counter. This is more complex than it needs to be.

This commit therefore simplifies things and saves a few lines of code by dispensing with the atomic decrements in favor of having trc_read_check_handler() do the atomic increment only in the success case. In theory, this represents no change in functionality.

Signed-off-by: Paul E. McKenney paulmck@kernel.org Cc: stable@vger.kernel.org # 5.15.x Signed-off-by: Joel Fernandes (Google) joel@joelfernandes.org --- I confirmed the patch fixes the following splat which happens twice on TRACE02 rcutorture test:

[ 765.941351] WARNING: CPU: 0 PID: 80 at kernel/rcu/tasks.h:895 trc_read_check_handler+0x61/0xe0 [ 765.949880] Modules linked in: [ 765.953006] CPU: 0 PID: 80 Comm: rcu_torture_rea Not tainted 5.15.86-rc1+ #25 [ 765.959982] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.15.0-1 04/01/2014 [ 765.967964] RIP: 0010:trc_read_check_handler+0x61/0xe0 [ 765.973050] Code: 01 00 89 c0 48 03 2c c5 80 f8 a5 ae c6 45 00 00 [..] [ 765.991768] RSP: 0000:ffffa64ac0003fb0 EFLAGS: 00010047 [ 765.997042] RAX: ffffffffad4f8610 RBX: ffffa26b41bd3000 RCX: ffffa26b5f4ac8c0 [ 766.004418] RDX: 0000000000000000 RSI: ffffffffae978121 RDI: ffffa26b41bd3000 [ 766.011502] RBP: ffffa26b41bd6000 R08: ffffa26b41bd3000 R09: 0000000000000000 [ 766.018778] R10: 0000000000000000 R11: ffffa64ac0003ff8 R12: 0000000000000000 [ 766.025943] R13: ffffa26b5f4ac8c0 R14: 0000000000000000 R15: 0000000000000000 [ 766.034383] FS: 0000000000000000(0000) GS:ffffa26b5f400000(0000) knlGS:0000000000000000 [ 766.042925] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 766.048775] CR2: 0000000000000000 CR3: 0000000001924000 CR4: 00000000000006f0 [ 766.055991] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 [ 766.063135] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 [ 766.070711] Call Trace: [ 766.073515] <IRQ> [ 766.075807] flush_smp_call_function_queue+0xec/0x1a0 [ 766.081087] __sysvec_call_function_single+0x3e/0x1d0 [ 766.086466] sysvec_call_function_single+0x89/0xc0 [ 766.091431] </IRQ> [ 766.093713] <TASK> [ 766.095930] asm_sysvec_call_function_single+0x16/0x20

kernel/rcu/tasks.h | 20 +++----------------- 1 file changed, 3 insertions(+), 17 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h index ae8396032b5d..4bd07cc3c0ea 100644 --- a/kernel/rcu/tasks.h +++ b/kernel/rcu/tasks.h @@ -892,32 +892,24 @@ static void trc_read_check_handler(void *t_in)

// If the task is no longer running on this CPU, leave. if (unlikely(texp != t)) { - if (WARN_ON_ONCE(atomic_dec_and_test(&trc_n_readers_need_end))) - wake_up(&trc_wait); goto reset_ipi; // Already on holdout list, so will check later. }

// If the task is not in a read-side critical section, and // if this is the last reader, awaken the grace-period kthread. if (likely(!READ_ONCE(t->trc_reader_nesting))) { - if (WARN_ON_ONCE(atomic_dec_and_test(&trc_n_readers_need_end))) - wake_up(&trc_wait); - // Mark as checked after decrement to avoid false - // positives on the above WARN_ON_ONCE(). WRITE_ONCE(t->trc_reader_checked, true); goto reset_ipi; } // If we are racing with an rcu_read_unlock_trace(), try again later. - if (unlikely(READ_ONCE(t->trc_reader_nesting) < 0)) { - if (WARN_ON_ONCE(atomic_dec_and_test(&trc_n_readers_need_end))) - wake_up(&trc_wait); + if (unlikely(READ_ONCE(t->trc_reader_nesting) < 0)) goto reset_ipi; - } WRITE_ONCE(t->trc_reader_checked, true);

// Get here if the task is in a read-side critical section. Set // its state so that it will awaken the grace-period kthread upon // exit from that critical section. + atomic_inc(&trc_n_readers_need_end); // One more to wait on. WARN_ON_ONCE(READ_ONCE(t->trc_reader_special.b.need_qs)); WRITE_ONCE(t->trc_reader_special.b.need_qs, true);

@@ -1017,21 +1009,15 @@ static void trc_wait_for_one_reader(struct task_struct *t, if (per_cpu(trc_ipi_to_cpu, cpu) || t->trc_ipi_to_cpu >= 0) return;

- atomic_inc(&trc_n_readers_need_end); per_cpu(trc_ipi_to_cpu, cpu) = true; t->trc_ipi_to_cpu = cpu; rcu_tasks_trace.n_ipis++; - if (smp_call_function_single(cpu, - trc_read_check_handler, t, 0)) { + if (smp_call_function_single(cpu, trc_read_check_handler, t, 0)) { // Just in case there is some other reason for // failure than the target CPU being offline. rcu_tasks_trace.n_ipis_fails++; per_cpu(trc_ipi_to_cpu, cpu) = false; t->trc_ipi_to_cpu = cpu; - if (atomic_dec_and_test(&trc_n_readers_need_end)) { - WARN_ON_ONCE(1); - wake_up(&trc_wait); - } } } }