[PATCH net] pds_core: yield the CPU while waiting for the adminq to drain

[PATCH net] pds_core: yield the CPU while waiting for the adminq to drain

[Nikhil P. Rao]

pdsc_adminq_wait_and_dec_once_unused() busy-waits for adminq_refcnt to
drop to one:

	while (!refcount_dec_if_one(&pdsc->adminq_refcnt))
		cpu_relax();

The refcount is held by pdsc_adminq_post() for the duration of an
in-flight command, which can wait up to devcmd_timeout seconds
(PDS_CORE_DEVCMD_TIMEOUT is 5) for the hardware to complete. cpu_relax()
is not a reschedule point, so on a non-preemptible kernel this loop can
spin on the CPU for several seconds, starving other tasks on that core.

Add cond_resched() to the loop so the waiter yields to other runnable
tasks while it polls, keeping cpu_relax() as the busy-wait hint between
checks.

Fixes: 7e82a8745b95 ("pds_core: Prevent race issues involving the adminq")
Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260629200358.2626129-1-nikhil.rao%40amd.com?part=2
Signed-off-by: Nikhil P. Rao <nikhil.rao@amd.com>
Reviewed-by: Eric Joyner <eric.joyner@amd.com>
---
 drivers/net/ethernet/amd/pds_core/core.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/net/ethernet/amd/pds_core/core.c b/drivers/net/ethernet/amd/pds_core/core.c
index 38a2446571af..19eeb5b06f4d 100644
--- a/drivers/net/ethernet/amd/pds_core/core.c
+++ b/drivers/net/ethernet/amd/pds_core/core.c
@@ -533,6 +533,7 @@ static void pdsc_adminq_wait_and_dec_once_unused(struct pdsc *pdsc)
 		dev_dbg_ratelimited(pdsc->dev, "%s: adminq in use\n",
 				    __func__);
 		cpu_relax();
+		cond_resched();
 	}
 }
 
-- 
2.43.0

Re: [PATCH net] pds_core: yield the CPU while waiting for the adminq to drain

[Pavan Chebbi]

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On Wed, Jul 15, 2026 at 1:46 AM Nikhil P. Rao <nikhil.rao@amd.com> wrote:
>
> pdsc_adminq_wait_and_dec_once_unused() busy-waits for adminq_refcnt to
> drop to one:
>
>         while (!refcount_dec_if_one(&pdsc->adminq_refcnt))
>                 cpu_relax();
>
> The refcount is held by pdsc_adminq_post() for the duration of an
> in-flight command, which can wait up to devcmd_timeout seconds
> (PDS_CORE_DEVCMD_TIMEOUT is 5) for the hardware to complete. cpu_relax()
> is not a reschedule point, so on a non-preemptible kernel this loop can
> spin on the CPU for several seconds, starving other tasks on that core.
>
> Add cond_resched() to the loop so the waiter yields to other runnable
> tasks while it polls, keeping cpu_relax() as the busy-wait hint between
> checks.
>
> Fixes: 7e82a8745b95 ("pds_core: Prevent race issues involving the adminq")
> Reported-by: sashiko-bot <sashiko-bot@kernel.org>
> Closes: https://sashiko.dev/#/patchset/20260629200358.2626129-1-nikhil.rao%40amd.com?part=2
> Signed-off-by: Nikhil P. Rao <nikhil.rao@amd.com>
> Reviewed-by: Eric Joyner <eric.joyner@amd.com>
> ---
>  drivers/net/ethernet/amd/pds_core/core.c | 1 +
>  1 file changed, 1 insertion(+)
>
Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>

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Re: [PATCH net] pds_core: yield the CPU while waiting for the adminq to drain

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Tue, 14 Jul 2026 20:14:56 +0000 you wrote:
> pdsc_adminq_wait_and_dec_once_unused() busy-waits for adminq_refcnt to
> drop to one:
> 
> 	while (!refcount_dec_if_one(&pdsc->adminq_refcnt))
> 		cpu_relax();
> 
> The refcount is held by pdsc_adminq_post() for the duration of an
> in-flight command, which can wait up to devcmd_timeout seconds
> (PDS_CORE_DEVCMD_TIMEOUT is 5) for the hardware to complete. cpu_relax()
> is not a reschedule point, so on a non-preemptible kernel this loop can
> spin on the CPU for several seconds, starving other tasks on that core.
> 
> [...]

Here is the summary with links:
  - [net] pds_core: yield the CPU while waiting for the adminq to drain
    https://git.kernel.org/netdev/net/c/a11f0b8a2042

You are awesome, thank you!
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