From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mailout3.rbg.tum.de (mailout3.rbg.tum.de [131.159.0.8]) by sourceware.org (Postfix) with ESMTPS id 8A719384D3D5 for ; Fri, 9 Dec 2022 17:34:25 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 8A719384D3D5 Authentication-Results: sourceware.org; dmarc=none (p=none dis=none) header.from=in.tum.de Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=in.tum.de Received: from mailrelay1.rbg.tum.de (mailrelay1.in.tum.de [131.159.254.14]) by mailout3.rbg.tum.de (Postfix) with ESMTPS id 1BA7F100110; Fri, 9 Dec 2022 18:34:20 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=in.tum.de; s=20220209; t=1670607260; bh=NIEpJSfVW1R8nWJjuKGsGZoCDnf9Z+JH1WxhoMZ3xng=; h=Date:From:Subject:To:Cc:References:In-Reply-To:From; b=HzzPYCQtLlYe8crFZOex47YR3PbNjBErYrDWbwvKWaW2Dz+5X3d1pq1j4HuzPJrKr GxJe8w7cxKW+XuSqFLnxLcCTChZXJBX57Rc0SxYiZYAiNFryw11EHxSHpYKs9Nla9k YOzfSlwuQuO57GSK86yo1qr+f2unauaZValtm2eijCIFHilqlKSqbOlQ0u+hlw8DbE uxX/uYQCxMQFUTSnoMDncWEpvsZUbxBf5mckXM9Y9oFMsMIDJoAS2Ho/YA1WJNlimN WUPUgW/Hitr3kopZbiMDpIYRlVbsKeLTCzitSpBIZ2HgcqFJ6eoNeKLQfa8gj6kQGK M2XhTaGjS5dyQ== Received: by mailrelay1.rbg.tum.de (Postfix, from userid 112) id 166CD1A8; Fri, 9 Dec 2022 18:34:20 +0100 (CET) Received: from mailrelay1.rbg.tum.de (localhost [127.0.0.1]) by mailrelay1.rbg.tum.de (Postfix) with ESMTP id A84041A5; Fri, 9 Dec 2022 18:34:19 +0100 (CET) Received: from mail.in.tum.de (mailproxy.in.tum.de [IPv6:2a09:80c0::78]) by mailrelay1.rbg.tum.de (Postfix) with ESMTPS id A08E41A1; Fri, 9 Dec 2022 18:34:19 +0100 (CET) Received: by mail.in.tum.de (Postfix, from userid 112) id 976304A01A8; Fri, 9 Dec 2022 18:34:19 +0100 (CET) Received: (Authenticated sender: neumann) by mail.in.tum.de (Postfix) with ESMTPSA id C457D4A0023; Fri, 9 Dec 2022 18:34:18 +0100 (CET) (Extended-Queue-bit xtech_uw@fff.in.tum.de) Message-ID: <7d18f085-ae46-138d-4f04-df5857b7b014@in.tum.de> Date: Fri, 9 Dec 2022 18:34:18 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.5.1 From: Thomas Neumann Subject: [PATCH] initialize fde objects lazily To: "gcc-patches@gcc.gnu.org" Cc: "H.J. Lu" , Jakub Jelinek , Tamar Christina , Jason Merrill , Jonathan Wakely , Florian Weimer References: <2a4776b9-9271-bb3c-a626-d5ec22dae6f3@in.tum.de> <91045a34-a534-4436-bb06-cac32d797a36@in.tum.de> <87sfibqu1s.fsf@oldenburg.str.redhat.com> Content-Language: en-US In-Reply-To: <87sfibqu1s.fsf@oldenburg.str.redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-11.0 required=5.0 tests=BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,GIT_PATCH_0,RCVD_IN_MSPIKE_H3,RCVD_IN_MSPIKE_WL,SPF_HELO_NONE,SPF_PASS,TXREP autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: When registering an unwind frame with __register_frame_info_bases we currently initialize that fde object eagerly. This has the advantage that it is immutable afterwards and we can safely access it from multiple threads, but it has the disadvantage that we pay the initialization cost even if the application never throws an exception. This commit changes the logic to initialize the objects lazily. The objects themselves are inserted into the b-tree when registering the frame, but the sorted fde_vector is not constructed yet. Only on the first time that an exception tries to pass through the registered code the object is initialized. We notice that with a double checking, first doing a relaxed load of the sorted bit and then re-checking under a mutex when the object was not initialized yet. Note that the check must implicitly be safe concering a concurrent frame deregistration, as trying the deregister a frame that is on the unwinding path of a concurrent exception is inherently racy. libgcc/ChangeLog: * unwind-dw2-fde.c: Initialize fde object lazily when the first exception tries to pass through. --- libgcc/unwind-dw2-fde.c | 52 ++++++++++++++++++++++++++++++++--------- 1 file changed, 41 insertions(+), 11 deletions(-) diff --git a/libgcc/unwind-dw2-fde.c b/libgcc/unwind-dw2-fde.c index 3c0cc654ec0..6f69c20ff4b 100644 --- a/libgcc/unwind-dw2-fde.c +++ b/libgcc/unwind-dw2-fde.c @@ -63,8 +63,6 @@ release_registered_frames (void) static void get_pc_range (const struct object *ob, uintptr_type *range); -static void -init_object (struct object *ob); #else /* Without fast path frame deregistration must always succeed. */ @@ -76,6 +74,7 @@ static const int in_shutdown = 0; by decreasing value of pc_begin. */ static struct object *unseen_objects; static struct object *seen_objects; +#endif #ifdef __GTHREAD_MUTEX_INIT static __gthread_mutex_t object_mutex = __GTHREAD_MUTEX_INIT; @@ -103,7 +102,6 @@ init_object_mutex_once (void) static __gthread_mutex_t object_mutex; #endif #endif -#endif /* Called from crtbegin.o to register the unwind info for an object. */ @@ -126,10 +124,7 @@ __register_frame_info_bases (const void *begin, struct object *ob, #endif #ifdef ATOMIC_FDE_FAST_PATH - // Initialize eagerly to avoid locking later - init_object (ob); - - // And register the frame + // Register the frame in the b-tree uintptr_type range[2]; get_pc_range (ob, range); btree_insert (®istered_frames, range[0], range[1] - range[0], ob); @@ -180,10 +175,7 @@ __register_frame_info_table_bases (void *begin, struct object *ob, ob->s.b.encoding = DW_EH_PE_omit; #ifdef ATOMIC_FDE_FAST_PATH - // Initialize eagerly to avoid locking later - init_object (ob); - - // And register the frame + // Register the frame in the b-tree uintptr_type range[2]; get_pc_range (ob, range); btree_insert (®istered_frames, range[0], range[1] - range[0], ob); @@ -892,7 +884,15 @@ init_object (struct object* ob) accu.linear->orig_data = ob->u.single; ob->u.sort = accu.linear; +#ifdef ATOMIC_FDE_FAST_PATH + // We must update the sorted bit with an atomic operation + struct object tmp; + tmp.s.b = ob->s.b; + tmp.s.b.sorted = 1; + __atomic_store (&(ob->s.b), &(tmp.s.b), __ATOMIC_SEQ_CST); +#else ob->s.b.sorted = 1; +#endif } #ifdef ATOMIC_FDE_FAST_PATH @@ -1130,6 +1130,21 @@ search_object (struct object* ob, void *pc) } } +#ifdef ATOMIC_FDE_FAST_PATH + +// Check if the object was already initialized +static inline bool +is_object_initialized (struct object *ob) +{ + // We have to use relaxed atomics for the read, which + // is a bit involved as we read from a bitfield + struct object tmp; + __atomic_load (&(ob->s.b), &(tmp.s.b), __ATOMIC_RELAXED); + return tmp.s.b.sorted; +} + +#endif + const fde * _Unwind_Find_FDE (void *pc, struct dwarf_eh_bases *bases) { @@ -1141,6 +1156,21 @@ _Unwind_Find_FDE (void *pc, struct dwarf_eh_bases *bases) if (!ob) return NULL; + // Initialize the object lazily + if (!is_object_initialized (ob)) + { + // Check again under mutex + init_object_mutex_once (); + __gthread_mutex_lock (&object_mutex); + + if (!ob->s.b.sorted) + { + init_object (ob); + } + + __gthread_mutex_unlock (&object_mutex); + } + f = search_object (ob, pc); #else -- 2.37.2