smp: Use release stores for csd_lock_record() state

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__csd_lock_record() publishes per-CPU diagnostic state (cur_csd,
cur_csd_func, cur_csd_info) that is consumed from a remote CPU by
csd_lock_wait_toolong() via smp_load_acquire(&cur_csd). To order the
matching cur_csd_func/cur_csd_info stores before the cur_csd
publication, the producer issues smp_wmb() before writing cur_csd;
to order the publication before the subsequent callback execution or
CSD unlock, it issues smp_mb() after the write. The clear path
mirrors this with smp_mb() before storing NULL into cur_csd so the
preceding callback/unlock is observed first.

The smp_mb() pair is heavier than what the consumer actually
requires (on x86 each emits a locked full barrier). The consumer
only needs to observe the matching cur_csd_func/cur_csd_info when it
sees a non-NULL cur_csd, and to observe the preceding callback/unlock
when it sees NULL -- both of which a release/acquire pair provides.
The extra two-way ordering enforced by smp_mb() -- that cur_csd
publication be observed before callback execution or unlock becomes
visible -- would only matter if cur_csd were an exact live-state
marker. csd_lock_wait_toolong() does not treat it that way: it
snapshots cur_csd via smp_load_acquire() and then prints / dumps /
re-IPIs without an RCU-style stall-ended recheck, so the diagnostic
already tolerates the remote CPU completing its work between snapshot
and report. cur_csd is best-effort context, not a precise stall
boundary.

Replace the smp_wmb() + plain store + smp_mb() in the publish path,
and the smp_mb() + plain store in the clear path, with
smp_store_release(). This pairs with the smp_load_acquire() in
csd_lock_wait_toolong(): preceding cur_csd_func/cur_csd_info stores
become visible before a remote reader observes the non-NULL
publication, and any preceding callback/unlock becomes visible before
a reader observes the NULL clear.

Signed-off-by: Usama Arif <usama.arif@linux.dev>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Dmitry Ilvokhin <d@ilvokhin.com>
Link: https://patch.msgid.link/20260629105745.1696683-1-usama.arif@linux.dev

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Usama Arif 2026-06-29 03:57:45 -07:00 • committed by Thomas Gleixner

parent dc59e4fea9

commit ec9f57e6fe

1 changed files with 19 additions and 7 deletions

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kernel/smp.c

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