From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 BEE563AA1A4; Thu, 5 Mar 2026 14:51:08 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772722268; cv=none; b=YtdhC6WpRBI+wzqIx7JaKD/p0ybmdKh/l9DzZGPhTiW5HvA0LDqEFDneXZRJ0LFUCY7xlR+k7E4s51LI3KG2VBtMpCr5HLaYeibv4hOb33c72/+cRHhtT8CsUPDEQS1kpmjkboRZ0sv4fyKRgp3EAxJpUDCQq4j/MzJDZ5Dqg6Q= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772722268; c=relaxed/simple; bh=z1MJC3FRweYmPEPq5YHZ5jyoBMGe/oJK14mlqTwgr2g=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=oTd2fhhCijYlekbnyRgEVYfq7uC21jQAFmk1dhcpdY6y+lZ5yjSIBtP2kP+MGAxY18sg0lg80moShWYNxqqdl3BfLa0sZqzR90THtO81h7PrkWSqRnpQbli1P7inKSYO8JNXJm0i5shMlJsYPR8vu9D1vc572JY80j8tgi2IzMw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=WCqUfgDt; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="WCqUfgDt" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 34A6DC2BCB2; Thu, 5 Mar 2026 14:51:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1772722268; bh=z1MJC3FRweYmPEPq5YHZ5jyoBMGe/oJK14mlqTwgr2g=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=WCqUfgDtAMRuBloIeMBdO5woK5MhRvEABy/vWszqjFiasJwr1FEBPGf0f9m8AcWTL DAde67hUsfYOPH27zL88wNZhNtBT2+7ACPnoZl1MgdFx90ZQcQn8hi5168Nmr/s+1x VgLYJqT6do7fWh2EMqaospMZ7wXf/ah/r1oOmyXRqbSaoejvXUj1NRpqeJOOZC3bTl W9QMj84j28rGJyIbhLnZ9dftosgBB8ot5czDpbo4MPsxZXi/PAdsZEXNYlUIEriFQJ AtcvdvFyL2f89IlhTAvWUMYaJnre8dETjS1vD2A7ERUuPKMNlHb2FdtdOqiV/7ZxCL U65Cy5p+6OxKg== From: Chuck Lever To: Anna Schumaker Cc: Trond Myklebust , , , Chuck Lever Subject: [PATCH v2 2/8] xprtrdma: Close sendctx get/put race that can block a transport Date: Thu, 5 Mar 2026 09:50:57 -0500 Message-ID: <20260305145054.7096-12-cel@kernel.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260305145054.7096-10-cel@kernel.org> References: <20260305145054.7096-10-cel@kernel.org> Precedence: bulk X-Mailing-List: linux-rdma@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-Developer-Signature: v=1; a=openpgp-sha256; l=2576; i=chuck.lever@oracle.com; h=from:subject; bh=Vj3diU24cdWhFoe5Txt6pOMwv2moztIOqmveQ9QcMSA=; b=owEBbQKS/ZANAwAKATNqszNvZn+XAcsmYgBpqZhVOKb0U0pFAM9Sh6N1f9tHKLUfISLAkXkA2 QBwL5FiF3yJAjMEAAEKAB0WIQQosuWwEobfJDzyPv4zarMzb2Z/lwUCaamYVQAKCRAzarMzb2Z/ l1JsD/9kKf+u9X3l2tWUdIRSqNc4lrN7FEKsX3eMM9G27ATvQzuFSAF3nbTe/mJtL5NgiXFTLOK WGQQ/pv7qYXA0/MTZBz6iinLJJvzmS3+DruEfH0CiOOGIUWFztxQyIpnJd215hXM02yIIbN+LTd 77sdq9RE7hK8H23DxiU1N9FsginCfnM7gJP8/suwlhOiCb0o2YORRNmVBUfoh83HWgFCmGw107k QuXhaIDkEeZm5sJp1pIPrGbn4q3nZc1Y/3tKBAKcR7XdIp83dnGNxOCjBaAf5HE+qLyBOEQeH2i Nwf2BGU0UV+BC1RfBG+5RcC2L8ots8vzeBEaDpia+vmqv1lEXCOgyHfifsnnUlgAJIj7uqqixP7 DFSUKnd7Kaiu0PodUNNOFpLf9aWBsmQAY/qTwE/QfjrlE9Dm9LS7TBVkMH4auzGWuqd4yINORxg oUyKlFGmmuWKn/48+oivy4X3onj5PLPc/bjER0f/jMg9rI8td7dhtgA6csNLMR3+nRop4aPgZcP Ivr/fD7DLbVxf4tVc0mrwDPiugibv/MAxbRZIUVrNSk17FwMYG3ILeUU5/Wyl2blKbJTD++keCp A7JqOOkrUovwEKoV5Dki/+s4/s23EXpksrDeTJBkzfrsbHh6rzSwWPhgBQYCaeERWyFEVRS2dFP XDk/RvipE3ODmXg== X-Developer-Key: i=chuck.lever@oracle.com; a=openpgp; fpr=28B2E5B01286DF243CF23EFE336AB3336F667F97 Content-Transfer-Encoding: 8bit From: Chuck Lever rpcrdma_sendctx_get_locked() and rpcrdma_sendctx_put_locked() can race in a way that leaves XPRT_WRITE_SPACE set permanently, blocking all further sends on the transport: get_locked put_locked (Send completion) ---------- -------------------------- read rb_sc_tail -> ring full advance rb_sc_tail xprt_write_space(): test_bit(WRITE_SPACE) -> not set, return set_bit(WRITE_SPACE) return NULL (-EAGAIN) After the sender releases XPRT_LOCKED, the release path refuses to wake the next task because XPRT_WRITE_SPACE is set. The sender retries, finds XPRT_WRITE_SPACE still set, and sleeps on xprt_sending. No further Send completions arrive to clear the flag because no new Sends can be posted. With nconnect, the stalled transport's share of congestion credits are never returned, starving the remaining transports as well. Fixes: 05eb06d86685 ("xprtrdma: Fix occasional transport deadlock") Signed-off-by: Chuck Lever --- net/sunrpc/xprtrdma/verbs.c | 17 ++++++++++++++++- 1 file changed, 16 insertions(+), 1 deletion(-) diff --git a/net/sunrpc/xprtrdma/verbs.c b/net/sunrpc/xprtrdma/verbs.c index b51a162885bb..90fd83f2d846 100644 --- a/net/sunrpc/xprtrdma/verbs.c +++ b/net/sunrpc/xprtrdma/verbs.c @@ -708,6 +708,18 @@ struct rpcrdma_sendctx *rpcrdma_sendctx_get_locked(struct rpcrdma_xprt *r_xprt) */ xprt_wait_for_buffer_space(&r_xprt->rx_xprt); r_xprt->rx_stats.empty_sendctx_q++; + + /* Recheck: a Send completion between the ring-empty test + * and the set_bit could cause its xprt_write_space() to + * miss, leaving XPRT_WRITE_SPACE set with a non-full ring. + * The smp_mb__after_atomic() pairs with smp_store_release() + * in rpcrdma_sendctx_put_locked(). + */ + smp_mb__after_atomic(); + next_head = rpcrdma_sendctx_next(buf, buf->rb_sc_head); + if (next_head != READ_ONCE(buf->rb_sc_tail)) + xprt_write_space(&r_xprt->rx_xprt); + return NULL; } @@ -739,7 +751,10 @@ static void rpcrdma_sendctx_put_locked(struct rpcrdma_xprt *r_xprt, } while (buf->rb_sc_ctxs[next_tail] != sc); - /* Paired with READ_ONCE */ + /* Paired with READ_ONCE in rpcrdma_sendctx_get_locked(): + * both the fast-path ring-full test and the post-set_bit + * recheck in the slow path depend on this store-release. + */ smp_store_release(&buf->rb_sc_tail, next_tail); xprt_write_space(&r_xprt->rx_xprt); -- 2.53.0