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smp: Make CSD lock acquisition atomic for debug mode

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Commit b0473dcd4b ("smp: Improve smp_call_function_single()
CSD-lock diagnostics") changed smp_call_function_single() so that,
when CSD lock debugging is enabled, async !wait calls use the
destination CPU csd_data. That improves diagnostics, but it also removes
the single-writer property that made the old csd_lock() safe: multiple
CPUs can now prepare the same destination CPU CSD concurrently.

csd_lock() currently waits for CSD_FLAG_LOCK to clear and then sets the
bit with a non-atomic read-modify-write. Two senders can both see an
unlocked CSD, set the bit, overwrite the callback fields, and enqueue
the same llist node. Re-adding a node that is already the queue head can
make node->next point to itself, leaving the target CPU stuck walking
call_single_queue. Later synchronous work, such as a TLB shootdown, can
then remain queued and trigger soft-lockup warnings or panics.

Keep the single csd_lock() implementation, but when CSD lock debugging is
enabled, acquire CSD_FLAG_LOCK with try_cmpxchg_acquire(). This makes the
destination CPU CSD a real atomic lock in the only configuration where it
can be shared by multiple remote senders, while preserving the existing
non-debug fast path.

Fixes: b0473dcd4b ("smp: Improve smp_call_function_single() CSD-lock diagnostics")
Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20260716004539.13983-2-paulmck@kernel.org
This commit is contained in:
Chuyi Zhou 2026-07-15 17:45:39 -07:00 committed by Thomas Gleixner
parent c58ea9adf7
commit 35551efb15
1 changed files with 23 additions and 4 deletions
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27
kernel/smp.c
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@ -137,10 +137,10 @@ csd_do_func(smp_call_func_t func, void *info, call_single_data_t *csd)
trace_csd_function_exit(func, csd);
}
#ifdef CONFIG_CSD_LOCK_WAIT_DEBUG
static DEFINE_STATIC_KEY_MAYBE(CONFIG_CSD_LOCK_WAIT_DEBUG_DEFAULT, csdlock_debug_enabled);
#ifdef CONFIG_CSD_LOCK_WAIT_DEBUG
/*
* Parse the csdlock_debug= kernel boot parameter.
*
@ -342,6 +342,10 @@ static __always_inline void csd_lock_wait(call_single_data_t *csd)
smp_cond_load_acquire(&csd->node.u_flags, !(VAL & CSD_FLAG_LOCK));
}
#else
static __always_inline void __csd_lock_wait(call_single_data_t *csd)
{
}
static void csd_lock_record(call_single_data_t *csd)
{
}
@ -354,8 +358,23 @@ static __always_inline void csd_lock_wait(call_single_data_t *csd)
static __always_inline void csd_lock(call_single_data_t *csd)
{
csd_lock_wait(csd);
csd->node.u_flags |= CSD_FLAG_LOCK;
if (IS_ENABLED(CONFIG_CSD_LOCK_WAIT_DEBUG) &&
static_branch_unlikely(&csdlock_debug_enabled)) {
for (;;) {
unsigned int flags;
__csd_lock_wait(csd);
flags = READ_ONCE(csd->node.u_flags);
if (!(flags & CSD_FLAG_LOCK) &&
try_cmpxchg_acquire(&csd->node.u_flags, &flags, flags | CSD_FLAG_LOCK))
break;
}
} else {
csd_lock_wait(csd);
csd->node.u_flags |= CSD_FLAG_LOCK;
}
/*
* prevent CPU from reordering the above assignment