Re: [tip: smp/core] smp: Use release stores for csd_lock_record() state

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jun 29, 2026 at 01:57:42PM -0000, tip-bot2 for Usama Arif wrote:
> The following commit has been merged into the smp/core branch of tip:
> 
> Commit-ID:     ec9f57e6fefbc0497b9f047ebe60ef9adaae7480
> Gitweb:        https://git.kernel.org/tip/ec9f57e6fefbc0497b9f047ebe60ef9adaae7480
> Author:        Usama Arif <usama.arif@linux.dev>
> AuthorDate:    Mon, 29 Jun 2026 03:57:45 -07:00
> Committer:     Thomas Gleixner <tglx@kernel.org>
> CommitterDate: Mon, 29 Jun 2026 15:53:14 +02:00
> 
> smp: Use release stores for csd_lock_record() state
> 
> __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
> ---
>  kernel/smp.c | 26 +++++++++++++++++++-------
>  1 file changed, 19 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/smp.c b/kernel/smp.c
> index a0bb56b..8a847a3 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -182,16 +182,22 @@ static atomic_t csd_bug_count = ATOMIC_INIT(0);
>  static void __csd_lock_record(call_single_data_t *csd)
>  {
>  	if (!csd) {
> -		smp_mb(); /* NULL cur_csd after unlock. */
> -		__this_cpu_write(cur_csd, NULL);
> +		/*
> +		 * Pairs with smp_load_acquire() of cur_csd in
> +		 * csd_lock_wait_toolong(): orders any preceding CSD
> +		 * callback/unlock before a remote reader observes NULL.
> +		 */
> +		smp_store_release(this_cpu_ptr(&cur_csd), NULL);

So at some point I suppose we should have *this_cpu_store_release() I
suppose. As is this generates suboptimal code on x86. But since I don't
typically run with CSD_LOCK_WAIT_DEBUG=y, I'll pretend not to care at
this exact moment.

>  		return;
>  	}
>  	__this_cpu_write(cur_csd_func, csd->func);
>  	__this_cpu_write(cur_csd_info, csd->info);
> -	smp_wmb(); /* func and info before csd. */
> -	__this_cpu_write(cur_csd, csd);
> -	smp_mb(); /* Update cur_csd before function call. */
> -		  /* Or before unlock, as the case may be. */
> +	/*
> +	 * Pairs with smp_load_acquire() of cur_csd in
> +	 * csd_lock_wait_toolong(): publishes cur_csd_func and
> +	 * cur_csd_info before the non-NULL pointer becomes visible.
> +	 */
> +	smp_store_release(this_cpu_ptr(&cur_csd), csd);

Let me say that RELEASE+ACQUIRE does not a full barrier make,
specifically smp_store_release() and smp_load_acquire() do not have
RCsc, they're RCtso or possibly weaker still (I don't remember).  The
thing that saves your bacon here is the address dependency.

>  }
>  static __always_inline void csd_lock_record(call_single_data_t *csd)
> @@ -272,7 +278,13 @@ static bool csd_lock_wait_toolong(call_single_data_t *csd, u64 ts0, u64 *ts1, in
>  		cpux = 0;
>  	else
>  		cpux = cpu;
> -	cpu_cur_csd = smp_load_acquire(&per_cpu(cur_csd, cpux)); /* Before func and info. */
> +	/*
> +	 * Pairs with smp_store_release() of cur_csd in __csd_lock_record():
> +	 * a non-NULL cur_csd here implies cur_csd_func and cur_csd_info
> +	 * are the matching publication; a NULL value is ordered after any
> +	 * preceding CSD callback/unlock on the remote CPU.
> +	 */
> +	cpu_cur_csd = smp_load_acquire(&per_cpu(cur_csd, cpux));
>  	/* How long since this CSD lock was stuck. */
>  	ts_delta = ts2 - ts0;
>  	pr_alert("csd: %s non-responsive CSD lock (#%d) on CPU#%d, waiting %lld ns for CPU#%02d %pS(%ps).\n",