From: Long Li leo.lilong@huawei.com
[ Upstream commit 5ca00634c8bbb2979c73465588f486b9632f5ed5 ]
The cache_detail structure uses a "nextcheck" field to control hash table scanning intervals. When a table scan begins, nextcheck is set to current time plus 1800 seconds. During scanning, if cache_detail is not empty and a cache entry's expiry time is earlier than the current nextcheck, the nextcheck is updated to that expiry time.
This mechanism ensures that: 1) Empty cache_details are scanned every 1800 seconds to avoid unnecessary scans 2) Non-empty cache_details are scanned based on the earliest expiry time found
However, when adding a new cache entry to an empty cache_detail, the nextcheck time was not being updated, remaining at 1800 seconds. This could delay cache cleanup for up to 1800 seconds, potentially blocking threads(such as nfsd) that are waiting for cache cleanup.
Fix this by updating the nextcheck time whenever a new cache entry is added.
Signed-off-by: Long Li leo.lilong@huawei.com Signed-off-by: Chuck Lever chuck.lever@oracle.com Signed-off-by: Sasha Levin sashal@kernel.org ---
Based on my analysis of the commit, the code changes, and the kernel repository context, here is my assessment: **YES** This commit should be backported to stable kernel trees. Here's my extensive analysis: ## Analysis of the Problem The commit addresses a specific cache management issue in the SUNRPC subsystem that can cause significant delays (up to 1800 seconds/30 minutes) in cache cleanup operations. The problem occurs when: 1. A cache_detail starts empty, with nextcheck set to current time + 1800 seconds 2. A new cache entry is added to this empty cache_detail 3. The nextcheck time is NOT updated to reflect the new entry's expiry time 4. This means cache cleanup won't occur until the original 1800-second timeout expires 5. During this delay, threads like nfsd that depend on cache cleanup can be blocked ## Code Change Analysis The fix is minimal and surgical: ```c + if (detail->nextcheck > new->expiry_time) + detail->nextcheck = new->expiry_time + 1; ``` This 2-line addition in `sunrpc_cache_add_entry()` ensures that when a new cache entry is added, if its expiry time is earlier than the current nextcheck time, nextcheck is updated to trigger cleanup at the appropriate time. ## Why This Qualifies for Stable Backporting **1. Bug Fix Nature**: This clearly fixes a bug where cache cleanup could be inappropriately delayed, affecting system functionality. **2. Minimal Risk**: The change is extremely small and contained. It only adds a simple conditional check that makes the cache cleanup more responsive. There's no architectural change or complex logic. **3. Critical Subsystem Impact**: The SUNRPC cache affects NFS operations, which are critical for many production systems. A 1800-second delay in cache cleanup can severely impact NFS server responsiveness. **4. Clear User Impact**: The commit message explicitly mentions that this can block threads like nfsd, which directly affects users experiencing NFS service delays or hangs. **5. Historical Context**: Looking at the similar commits provided as reference, this fix is actually addressing a more fundamental timing issue than some of the other cache-related changes that were marked as "NO" for backporting. Those commits dealt with interface behavior changes or feature additions, while this fixes a clear performance/delay bug. **6. Low Complexity**: The fix doesn't introduce new features, change APIs, or modify behavioral contracts. It simply ensures that cache cleanup happens when it should happen. **7. Contained Scope**: The change is isolated to a single function (`sunrpc_cache_add_entry`) and only affects the timing of cache cleanup, not the cleanup logic itself. ## Comparison with Reference Commits Unlike the reference commits marked "NO" which involved: - Interface behavior changes (flush interface semantics) - Feature additions (new cache operations) - Complex timing logic modifications - Architectural changes This commit simply fixes a timing bug with a minimal, obvious correction that ensures cache cleanup occurs at the appropriate time rather than being unnecessarily delayed. The fix follows the stable tree rules perfectly: it's an important bugfix with minimal risk that addresses a clear user-visible issue in a critical kernel subsystem.
net/sunrpc/cache.c | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/net/sunrpc/cache.c b/net/sunrpc/cache.c index 95aab48d32e67..e8448e9e03d59 100644 --- a/net/sunrpc/cache.c +++ b/net/sunrpc/cache.c @@ -133,6 +133,8 @@ static struct cache_head *sunrpc_cache_add_entry(struct cache_detail *detail,
hlist_add_head_rcu(&new->cache_list, head); detail->entries++; + if (detail->nextcheck > new->expiry_time) + detail->nextcheck = new->expiry_time + 1; cache_get(new); spin_unlock(&detail->hash_lock);