linux-kernel - Re: [PATCH] riscv: trace: fix snapshot deadlock with sbi ecall

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Message-ID: <e06402bf-a612-4d9c-c5f6-5361826f5fdf@kernel.org>
Date: Wed, 7 Jan 2026 15:36:57 -0700 (MST)
From: Paul Walmsley <pjw@...nel.org>
To: Martin Kaiser <martin@...ser.cx>
cc: Paul Walmsley <pjw@...nel.org>, Palmer Dabbelt <palmer@...belt.com>, 
    linux-riscv@...ts.infradead.org, linux-trace-kernel@...r.kernel.org, 
    linux-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv: trace: fix snapshot deadlock with sbi ecall

On Tue, 23 Dec 2025, Martin Kaiser wrote:

> If sbi_ecall.c's functions are traceable,
> 
> echo "__sbi_ecall:snapshot" > /sys/kernel/tracing/set_ftrace_filter
> 
> may get the kernel into a deadlock.
> 
> (Functions in sbi_ecall.c are excluded from tracing if
> CONFIG_RISCV_ALTERNATIVE_EARLY is set.)
> 
> __sbi_ecall triggers a snapshot of the ringbuffer. The snapshot code
> raises an IPI interrupt, which results in another call to __sbi_ecall
> and another snapshot...
> 
> All it takes to get into this endless loop is one initial __sbi_ecall.
> On RISC-V systems without SSTC extension, the clock events in
> timer-riscv.c issue periodic sbi ecalls, making the problem easy to
> trigger.
> 
> Always exclude the sbi_ecall.c functions from tracing to fix the
> potential deadlock.
> 
> sbi ecalls can easiliy be logged via trace events, excluding ecall
> functions from function tracing is not a big limitation.
> 
> Signed-off-by: Martin Kaiser <martin@...ser.cx>

Thanks, queued for v6.19-rc.

- Paul