From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 299CE3B8BBB; Sat, 17 Oct 2026 17:26:42 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792258003; cv=none; b=qWRnf0lNGREzfyChcEPAsfXtUWle92xEZROwfwyZzApkJ6k6H/bZ1cY6yz8sl1NlmT2U53eedEEr8BzIrlxLrHX9IIC3ja1LqCp2dprIvQNVpmy+vS2C8cnVoC0IRAuPOYJ06hJ9Vga23pnprtFLpGj4IRLYqL0Aj8xcz8T2CPQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792258003; c=relaxed/simple; bh=kiZmJkZ+Z/wrC74CycG1z/LvfOjgeYlqGHzFfFT7s2c=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=hA+dh4ytVNxhQ9vtZprvcZ9YrymXx9Hk40JQk3oFxE5zet9US8lOHAkGpA14CxHLsHAtqTZsx3yDaI6rhMbrwSACtm7Jqyt58SGAa5zMHph7hraDMMNYh6RcawoC7FXWh6ZVRFQLcwu3wAst6ehGanCx8E38kdECafHkJeEcF3Q= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=1tSqxQiZ; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="1tSqxQiZ" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 81D361F000C5; Sat, 17 Oct 2026 17:26:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1792258002; bh=NIlf9Rndjrp9Dve4CkFCxetYT9B1/el1Y/RE3H8J9Y4=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=1tSqxQiZYFldSrLgUFqcSHmPKvxpdIRZetMILxjmFa17y+ZgPvZtKS8kVaI2qDv8Q wU/LP5/ylHkkIZrHduxTvoggOWQbsavJBkGI/zJMrsqBsOWKVKH116zRgQDvSMXuNO LwqoCvNStyvdTSPCs6CwDPD8Fw6tGjXUhz4AP8ZE= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Johannes Berg , Richard Weinberger , Sasha Levin Subject: [PATCH 6.6 0739/1674] um: fix time-travel syscall scheduling hack Date: Sat, 17 Oct 2026 17:50:11 +0200 Message-ID: <20261017160450.067962706@linuxfoundation.org> X-Mailer: git-send-email 2.56.0 In-Reply-To: <20261017160431.119607952@linuxfoundation.org> References: <20261017160431.119607952@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.6-stable review patch. If anyone has any objections, please let me know. ------------------ From: Johannes Berg [ Upstream commit 381d2f95c8aa575d5d42bf1fe0ea9a70c4bec0cf ] The schedule() call there really never did anything at least since the introduction of the EEVDF scheduler, but now I found a case where we permanently hang in a loop of -ERESTARTNOINTR (due to locking.) Work around it by making any syscalls with error return take time (and then schedule after) so we cannot hang in such a loop forever. Signed-off-by: Johannes Berg Signed-off-by: Richard Weinberger Stable-dep-of: 887c5c12e80c ("um: work around sched_yield not yielding in time-travel mode") Signed-off-by: Sasha Levin --- arch/um/kernel/skas/syscall.c | 34 ++++++++++++++++++++-------------- 1 file changed, 20 insertions(+), 14 deletions(-) diff --git a/arch/um/kernel/skas/syscall.c b/arch/um/kernel/skas/syscall.c index 9ee19e566da3b..b09e85279d2b8 100644 --- a/arch/um/kernel/skas/syscall.c +++ b/arch/um/kernel/skas/syscall.c @@ -12,23 +12,13 @@ #include #include #include +#include void handle_syscall(struct uml_pt_regs *r) { struct pt_regs *regs = container_of(r, struct pt_regs, regs); int syscall; - /* - * If we have infinite CPU resources, then make every syscall also a - * preemption point, since we don't have any other preemption in this - * case, and kernel threads would basically never run until userspace - * went to sleep, even if said userspace interacts with the kernel in - * various ways. - */ - if (time_travel_mode == TT_MODE_INFCPU || - time_travel_mode == TT_MODE_EXTERNAL) - schedule(); - /* Initialize the syscall number and default return value. */ UPT_SYSCALL_NR(r) = PT_SYSCALL_NR(r->gp); PT_REGS_SET_SYSCALL_RETURN(regs, -ENOSYS); @@ -41,9 +31,25 @@ void handle_syscall(struct uml_pt_regs *r) goto out; syscall = UPT_SYSCALL_NR(r); - if (syscall >= 0 && syscall < __NR_syscalls) - PT_REGS_SET_SYSCALL_RETURN(regs, - EXECUTE_SYSCALL(syscall, regs)); + if (syscall >= 0 && syscall < __NR_syscalls) { + unsigned long ret = EXECUTE_SYSCALL(syscall, regs); + + PT_REGS_SET_SYSCALL_RETURN(regs, ret); + + /* + * An error value here can be some form of -ERESTARTSYS + * and then we'd just loop. Make any error syscalls take + * some time, so that it won't just loop if something is + * not ready, and hopefully other things will make some + * progress. + */ + if (IS_ERR_VALUE(ret) && + (time_travel_mode == TT_MODE_INFCPU || + time_travel_mode == TT_MODE_EXTERNAL)) { + um_udelay(1); + schedule(); + } + } out: syscall_trace_leave(regs); -- 2.53.0