From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-182.mta1.migadu.com (out-182.mta1.migadu.com [95.215.58.182]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 7436918E1F for ; Thu, 6 Nov 2025 05:12:52 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.182 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762405975; cv=none; b=iRTuCOG25vvTUen/0hp11ZBkVy2Ou+ep3Qa4jyfaZqt94AIXH+dg6bUj7LPzMhPnOmuisnfWRWbd1XAphM69HR7bsjAI+s+K74RUzz8hzdjzyWkG3Ia5QKeVbFcaskH1b0pPjdH8RvWECt5l57vRh6PBJDSn4CcFS2zEBR7ECd8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762405975; c=relaxed/simple; bh=dWTRRhraEehfMjgHXaXWo7OBff76j3I9dEbnTVHSZLs=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=pAxtJVeFi3VNpUvKx9dxqBbG4YbqgxcVfbRB2ycz/iMiAR5ZGijtnv01dtan2J61lB50RMYlDWflI+ALUeWbFLxQpSPul6OiIkNHThkY8OsAl2beUzbhjauOwOJX40hakIQi8bWLXrwH78X/SykKN0KPKj+tI7jk39B+HKfUuQg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=DrU/lOXa; arc=none smtp.client-ip=95.215.58.182 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="DrU/lOXa" Message-ID: <363717bf-499a-4e47-b2c9-8a6e4105282c@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1762405970; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=I/qhxXZz0JAIUm3sRMPspbyF1ft2vsLDuBxnlxXhzwM=; b=DrU/lOXaxAyRoa3slmTMahv1AhNm6YOLXRkemOflIJCWqQTLd/Uj4zBg02sKXSBBytSpdr iRytZXNVWSrYUwFu1jTYx+8CYRoR1PGqrSK+e2tF3Ljt2mLftlcDKrTGxJGiq/FAzL6oyu qVUHNyn8Xz1mLMhx+4xNknET5Bf36xA= Date: Thu, 6 Nov 2025 13:12:24 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH bpf-next v4 2/2] bpf: Hold the perf callchain entry until used completely To: Yonghong Song , peterz@infradead.org, mingo@redhat.com, acme@kernel.org, namhyung@kernel.org, mark.rutland@arm.com, alexander.shishkin@linux.intel.com, jolsa@kernel.org, irogers@google.com, adrian.hunter@intel.com, kan.liang@linux.intel.com, song@kernel.org, ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, martin.lau@linux.dev, eddyz87@gmail.com, john.fastabend@gmail.com, kpsingh@kernel.org, sdf@fomichev.me, haoluo@google.com Cc: linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, bpf@vger.kernel.org References: <20251028162502.3418817-1-chen.dylane@linux.dev> <20251028162502.3418817-3-chen.dylane@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Tao Chen In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT 在 2025/11/6 06:16, Yonghong Song 写道: > > > On 10/28/25 9:25 AM, Tao Chen wrote: >> As Alexei noted, get_perf_callchain() return values may be reused >> if a task is preempted after the BPF program enters migrate disable >> mode. The perf_callchain_entres has a small stack of entries, and >> we can reuse it as follows: >> >> 1. get the perf callchain entry >> 2. BPF use... >> 3. put the perf callchain entry >> >> Signed-off-by: Tao Chen >> --- >>   kernel/bpf/stackmap.c | 61 ++++++++++++++++++++++++++++++++++--------- >>   1 file changed, 48 insertions(+), 13 deletions(-) >> >> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c >> index e28b35c7e0b..70d38249083 100644 >> --- a/kernel/bpf/stackmap.c >> +++ b/kernel/bpf/stackmap.c >> @@ -188,13 +188,12 @@ static void stack_map_get_build_id_offset(struct >> bpf_stack_build_id *id_offs, >>   } >>   static struct perf_callchain_entry * >> -get_callchain_entry_for_task(struct task_struct *task, u32 max_depth) >> +get_callchain_entry_for_task(int *rctx, struct task_struct *task, u32 >> max_depth) >>   { >>   #ifdef CONFIG_STACKTRACE >>       struct perf_callchain_entry *entry; >> -    int rctx; >> -    entry = get_callchain_entry(&rctx); >> +    entry = get_callchain_entry(rctx); >>       if (!entry) >>           return NULL; >> @@ -216,8 +215,6 @@ get_callchain_entry_for_task(struct task_struct >> *task, u32 max_depth) >>               to[i] = (u64)(from[i]); >>       } >> -    put_callchain_entry(rctx); >> - >>       return entry; >>   #else /* CONFIG_STACKTRACE */ >>       return NULL; >> @@ -297,6 +294,31 @@ static long __bpf_get_stackid(struct bpf_map *map, >>       return id; >>   } >> +static struct perf_callchain_entry * >> +bpf_get_perf_callchain(int *rctx, struct pt_regs *regs, bool kernel, >> bool user, >> +               int max_stack, bool crosstask) >> +{ >> +    struct perf_callchain_entry_ctx ctx; >> +    struct perf_callchain_entry *entry; >> + >> +    entry = get_callchain_entry(rctx); > > I think this may not work. Let us say we have two bpf programs > both pinned to a particular cpu (migrate disabled but preempt enabled). > get_callchain_entry() calls get_recursion_context() to get the > buffer for a particulart level. > > static inline int get_recursion_context(u8 *recursion) > { >         unsigned char rctx = interrupt_context_level(); >         if (recursion[rctx]) >                 return -1; >         recursion[rctx]++; >         barrier(); >         return rctx; > } > > It is possible that both tasks (at process level) may > reach right before "recursion[rctx]++;". > In such cases, both tasks will be able to get > buffer and this is not right. > > To fix this, we either need to have preempt disable > in bpf side, or maybe we have some kind of atomic > operation (cmpxchg or similar things), or maybe > has a preempt disable between if statement and recursion[rctx]++, > so only one task can get buffer? > Thanks to your reminder, can we add preempt disable before and after get_callchain_entry, avoid affecting the original functions of perf. Regarding multiple task preemption: if the entry is not released via put_callchain_entry, it appears that perf's buffer does not support recording the second task, so it returns directly here. if (recursion[rctx]) return -1; > >> +    if (unlikely(!entry)) >> +        return NULL; >> + >> +    __init_perf_callchain_ctx(&ctx, entry, max_stack, false); >> +    if (kernel) >> +        __get_perf_callchain_kernel(&ctx, regs); >> +    if (user && !crosstask) >> +        __get_perf_callchain_user(&ctx, regs); >> + >> +    return entry; >> +} >> + >> +static void bpf_put_callchain_entry(int rctx) > > we have bpf_get_perf_callchain(), maybe rename the above > to bpf_put_perf_callchain()? > Ack, thanks. >> +{ >> +    put_callchain_entry(rctx); >> +} >> + > > [...] > -- Best Regards Tao Chen