Message-Id: <20250601232435.3507697-57-sashal@kernel.org>
Date: Sun,  1 Jun 2025 19:23:39 -0400
From: Sasha Levin <sashal@...nel.org>
To: patches@...ts.linux.dev,
	stable@...r.kernel.org
Cc: Long Li <leo.lilong@...wei.com>,
	Chuck Lever <chuck.lever@...cle.com>,
	Sasha Levin <sashal@...nel.org>,
	jlayton@...nel.org,
	trondmy@...nel.org,
	anna@...nel.org,
	davem@...emloft.net,
	edumazet@...gle.com,
	kuba@...nel.org,
	pabeni@...hat.com,
	linux-nfs@...r.kernel.org,
	netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH AUTOSEL 6.15 057/110] sunrpc: fix race in cache cleanup causing stale nextcheck time

From: Long Li <leo.lilong@...wei.com>

[ Upstream commit 2298abcbe11e9b553d03c0f1d084da786f7eff88 ]

When cache cleanup runs concurrently with cache entry removal, a race
condition can occur that leads to incorrect nextcheck times. This can
delay cache cleanup for the cache_detail by up to 1800 seconds:

1. cache_clean() sets nextcheck to current time plus 1800 seconds
2. While scanning a non-empty bucket, concurrent cache entry removal can
   empty that bucket
3. cache_clean() finds no cache entries in the now-empty bucket to update
   the nextcheck time
4. This maybe delays the next scan of the cache_detail by up to 1800
   seconds even when it should be scanned earlier based on remaining
   entries

Fix this by moving the hash_lock acquisition earlier in cache_clean().
This ensures bucket emptiness checks and nextcheck updates happen
atomically, preventing the race between cleanup and entry removal.

Signed-off-by: Long Li <leo.lilong@...wei.com>
Signed-off-by: Chuck Lever <chuck.lever@...cle.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---

Now let me analyze this commit in detail based on what I've learned:
**ANALYSIS:** **YES** - This commit should be backported to stable
kernel trees. ## Detailed Analysis: ### 1. **Bug Impact Assessment**
This commit fixes a **race condition** that can cause cache cleanup
delays of up to **1800 seconds (30 minutes)**. This is a significant
performance regression that affects the SUNRPC cache subsystem's ability
to clean expired entries in a timely manner. ### 2. **Root Cause
Analysis** The race condition occurs in the `cache_clean()` function
between these steps: - **Line 461**: `current_detail->nextcheck =
seconds_since_boot()+30*60;` (sets nextcheck to current time + 1800
seconds) - **Lines 466-469**: The code searches for non-empty buckets
**without holding hash_lock** - **Lines 479-496**: The code then
acquires `hash_lock` and processes entries, updating nextcheck based on
entry expiry times **The race window**: Between lines 466-469 and 479,
concurrent cache operations (like `sunrpc_cache_unhash()`) can remove
entries from buckets, making them empty. When `cache_clean()` later
acquires the hash_lock, it finds empty buckets and cannot update the
nextcheck time appropriately. ### 3. **Code Change Analysis** The fix is
**minimal and surgical**: ```c // BEFORE: hash_lock acquired late (line
479) if (current_detail && current_index < current_detail->hash_size) {
// ... variable declarations ... spin_lock(&current_detail->hash_lock);
// TOO LATE! // AFTER: hash_lock acquired early (before bucket search)
spin_lock(&current_detail->hash_lock); while (current_index <
current_detail->hash_size &&
hlist_empty(&current_detail->hash_table[current_index]))
current_index++; ``` This ensures that: 1. **Atomicity**: Bucket
emptiness checks and nextcheck updates happen atomically 2. **Race
prevention**: No concurrent entry removal can happen during bucket
scanning 3. **Correctness**: nextcheck times are updated based on the
actual state of buckets at scan time ### 4. **Risk Assessment** **Low
Risk**: - **Scope**: Changes only affect the locking order in
`cache_clean()` - **Backwards compatibility**: No API changes - **Side
effects**: Minimal - just holds the lock slightly longer, which is safe
- **Testing**: The change preserves existing logic while fixing the race
### 5. **Stable Tree Criteria Compliance** ✅ **Fixes important bug**:
30-minute cache cleanup delays significantly impact performance ✅
**Small and contained**: Only 12 lines changed in a single function ✅
**No architectural changes**: Just reorders lock acquisition ✅ **Minimal
regression risk**: The change is conservative and preserves existing
behavior ✅ **Clear side effects**: Only improves correctness, no
negative impacts ### 6. **Comparison with Similar Commits** Looking at
the historical examples: - **Similar to Commit #3 (YES)**: Fixes a race
condition with minimal code changes and clear correctness improvement -
**Unlike Commits #1,#2,#4,#5 (NO)**: Those were either reverted,
cosmetic, or changed behavior significantly ### 7. **Critical System
Impact** SUNRPC cache is used by: - **NFS client/server operations** -
**RPC authentication caching** - **Various network filesystem
operations** A 30-minute delay in cache cleanup can cause: - Memory
bloat from stale entries - Performance degradation - Potential service
disruptions **Conclusion**: This is a clear bug fix with minimal risk
that addresses a significant performance issue in a critical kernel
subsystem. It meets all criteria for stable tree backporting.

 net/sunrpc/cache.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/net/sunrpc/cache.c b/net/sunrpc/cache.c
index bbaa77d7bbc81..131090f31e6a8 100644
--- a/net/sunrpc/cache.c
+++ b/net/sunrpc/cache.c
@@ -464,24 +464,21 @@ static int cache_clean(void)
 		}
 	}
 
+	spin_lock(&current_detail->hash_lock);
+
 	/* find a non-empty bucket in the table */
-	while (current_detail &&
-	       current_index < current_detail->hash_size &&
+	while (current_index < current_detail->hash_size &&
 	       hlist_empty(&current_detail->hash_table[current_index]))
 		current_index++;
 
 	/* find a cleanable entry in the bucket and clean it, or set to next bucket */
-
-	if (current_detail && current_index < current_detail->hash_size) {
+	if (current_index < current_detail->hash_size) {
 		struct cache_head *ch = NULL;
 		struct cache_detail *d;
 		struct hlist_head *head;
 		struct hlist_node *tmp;
 
-		spin_lock(&current_detail->hash_lock);
-
 		/* Ok, now to clean this strand */
-
 		head = &current_detail->hash_table[current_index];
 		hlist_for_each_entry_safe(ch, tmp, head, cache_list) {
 			if (current_detail->nextcheck > ch->expiry_time)
@@ -502,8 +499,10 @@ static int cache_clean(void)
 		spin_unlock(&cache_list_lock);
 		if (ch)
 			sunrpc_end_cache_remove_entry(ch, d);
-	} else
+	} else {
+		spin_unlock(&current_detail->hash_lock);
 		spin_unlock(&cache_list_lock);
+	}
 
 	return rv;
 }
-- 
2.39.5

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