From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) (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 5D3641E1022 for ; Wed, 2 Apr 2025 09:15:21 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.50.34 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1743585323; cv=none; b=dWct8XS3eR5Uyv5eJ5H87AIqZRXA3BJ5jLNHDF5IXU3ZuedaeiBvo4vkyjJzhT35aftKvy3XcThcB2DzCTwkgvxCCdFsO17ZFP7r0FdQrYjedto727k72/LjgxuU14Isg2mtVu22QoHKCQNuNgl1mM0S+BJlxehNQ2G3HM1wrXA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1743585323; c=relaxed/simple; bh=n/RxIaaccgNHBVClISeM0VdhOGZQNwC9h1yNM6a7Qkg=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Oj2RXOD/C9lagNnjgqscjCqb0FcZwxLNenvC/3/xLzoY8b9dGgw+6mPaUtGXyncgRaO3isOBLD7FDVgCucOe6qrmbC4bHYWO+e6Y/EozwQBKfSHH4+kmYkQwPtwvqOIv8MX9z75oTcvkbgaWKUJiOAMw0h8KgBOV+Jf93aLeWhQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=EigNL30l; arc=none smtp.client-ip=90.155.50.34 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="EigNL30l" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=vq7+ZxmIAUqYCHH83Qzf9D4wDj/DFpbKgAvaJzR7RZI=; b=EigNL30lw4h+Mv8YDvLQ8Iknkb gSrnjpUE22e3kFHGSOg7AMAcn49pCyDx4Y3fXZurpyel2OVWmIiBGwPLC1MKTYQ7nDGj21h5/Qwat q+jOWr2/0s0LC4KQGNV0balJChOouv30V9x+b1Ipn57l4GmW/TwzcsdEl5ptc3MxFAO6uRBn2Nt7j r27z95vxcchg1HQmPBPKE4YyzQbaayhiC6fM5pdw98G6mX7z4NGb9ERtHH8XcKyjOfSQvtrOaaIc4 8xaaBm5spb+Ppr/kcOvooV3g6ayhlCNyYOuc+jpVZpgr35XP8qVGQw1BfaEjjnL/qDIkckAQdqOEp fZVj6iQQ==; Received: from 77-249-17-252.cable.dynamic.v4.ziggo.nl ([77.249.17.252] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.98.1 #2 (Red Hat Linux)) id 1tzuBl-000000099oB-22jQ; Wed, 02 Apr 2025 09:15:13 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id D81E730049D; Wed, 2 Apr 2025 11:15:12 +0200 (CEST) Date: Wed, 2 Apr 2025 11:15:12 +0200 From: Peter Zijlstra To: Ravi Bangoria Cc: mingo@kernel.org, lucas.demarchi@intel.com, linux-kernel@vger.kernel.org, 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 Subject: Re: [PATCH v3 3/7] perf: Simplify perf_event_free_task() wait Message-ID: <20250402091512.GV5880@noisy.programming.kicks-ass.net> References: <20250307193305.486326750@infradead.org> <20250307193723.044499344@infradead.org> <1d9d8520-bb4d-4af3-a395-4a4e594e67e8@amd.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1d9d8520-bb4d-4af3-a395-4a4e594e67e8@amd.com> On Mon, Mar 17, 2025 at 12:19:07PM +0530, Ravi Bangoria wrote: > Hi Peter, > > > @@ -1223,8 +1223,14 @@ static void put_ctx(struct perf_event_co > > if (refcount_dec_and_test(&ctx->refcount)) { > > if (ctx->parent_ctx) > > put_ctx(ctx->parent_ctx); > > - if (ctx->task && ctx->task != TASK_TOMBSTONE) > > - put_task_struct(ctx->task); > > + if (ctx->task) { > > + if (ctx->task == TASK_TOMBSTONE) { > > + smp_mb(); /* pairs with wait_var_event() */ > > + wake_up_var(&ctx->refcount); > > perf_event_free_task() waits on "ctx->refcount == 1". But moving > wake_up_var() under refcount_dec_and_test() will cause > perf_event_free_task() to wait indefinitely. Right? So, shouldn't > wake_up_var() be outside? something like: > > --- a/kernel/events/core.c > +++ b/kernel/events/core.c > @@ -1281,15 +1281,14 @@ static void put_ctx(struct perf_event_context *ctx) > if (refcount_dec_and_test(&ctx->refcount)) { > if (ctx->parent_ctx) > put_ctx(ctx->parent_ctx); > - if (ctx->task) { > - if (ctx->task == TASK_TOMBSTONE) { > - smp_mb(); /* pairs with wait_var_event() */ > - wake_up_var(&ctx->refcount); > - } else { > - put_task_struct(ctx->task); > - } > - } > + if (ctx->task && ctx->task != TASK_TOMBSTONE) > + put_task_struct(ctx->task); > call_rcu(&ctx->rcu_head, free_ctx); > + } else { > + if (ctx->task == TASK_TOMBSTONE) { > + smp_mb(); /* pairs with wait_var_event() */ > + wake_up_var(&ctx->refcount); > + } > } > } Yes, you're quite right indeed. Thanks!