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Thu, 14 Nov 2024 12:16:44 +0000 Received: from BLAPR10MB5267.namprd10.prod.outlook.com ([fe80::682b:c879:9f97:a34f]) by BLAPR10MB5267.namprd10.prod.outlook.com ([fe80::682b:c879:9f97:a34f%3]) with mapi id 15.20.8158.017; Thu, 14 Nov 2024 12:16:44 +0000 Message-ID: <9bfe242b-b09b-47a5-9446-1cfc0897aef2@oracle.com> Date: Thu, 14 Nov 2024 12:16:40 +0000 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH dwarves 3/3] dwarf_loader: Check DW_OP_[GNU_]entry_value for possible parameter matching To: Yonghong Song , Arnaldo Carvalho de Melo Cc: Arnaldo Carvalho de Melo , dwarves@vger.kernel.org, Alexei Starovoitov , Andrii Nakryiko , bpf@vger.kernel.org, Daniel Borkmann , kernel-team@fb.com, Song Liu References: <20241108180508.1196431-1-yonghong.song@linux.dev> <20241108180524.1198900-1-yonghong.song@linux.dev> <5be88704-1bb0-4332-8626-26e7c908184c@linux.dev> <48a2d5a2-38e0-4c36-90cc-122602ff6386@linux.dev> <5e640168-7753-413a-ab00-f297948e84ef@oracle.com> <71778df3-62a6-4b1d-9ccf-4a8eb0e23828@oracle.com> <548c7b6b-3b84-4053-baa7-72976731ab87@linux.dev> Content-Language: en-GB From: Alan Maguire In-Reply-To: <548c7b6b-3b84-4053-baa7-72976731ab87@linux.dev> Content-Type: text/plain; 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From Eduard's analysis, it seems like it >> is safer to take the libdw__lock around dwarf_getlocation(s), since >> multiple threads can access the CU location cache. I've tried tweaking >> Eduard's modification of Yonghong's original patch and adding a second >> patch to add locking; with these two patches applied >> >> - we see the desired behaviour where perf_event_read() is present in >> BTF; and >> - we don't see any segmentation faults after ~700 iterations where I saw >> one every 200 or so before >> >> Yonghong, Eduard - do these changes look okay from your side? Feel free >> to resubmit if so (fixing up attributions as you see fit if they look >> wrong of course). Thanks! > > Thanks Alan for working on this. The following are some suggestions for > patch one: >   1. rename __dwarf_getlocations() to __parameter__locations()? >   2. rename param_reg_at_entry to parameter__locations()? Since it returns the register number, what about __parameter_reg/parameter_reg()? >   3. You missed the following: > static int param_reg_at_entry(Dwarf_Attribute *attr, int expected_reg) > { > ... >         if (first_expr)                     // this line >                 return first_expr->atom;    // this line >         return -1; > } > I _think_ I've preserved the behaviour described by the comment at the start without using the first_expr code. Note that we set "ret" in the "case DW_OP_reg0 ... DW_OP_reg31:" clause of the switch statement, so will return that value; either directly, if the register number matches expected reg, or eventually if we don't find any DW_OP_*entry_value location info to return. This I think matches the described behaviour: /* For DW_AT_location 'attr': * - if first location is DW_OP_regXX with expected number, returns the register; * - if location DW_OP_entry_value(DW_OP_regXX) is in the list, returns the register; * - if first location is DW_OP_regXX, returns the register; * - otherwise returns -1. */ ...but again I may have missed something here. > Patch 2 needs adjustment as well due to the above point #3. > Otherwise, LGTM. Since you are already preparing the patch, > please go ahead to pose v2 after you fixing the above things. > Sure; if the above sounds okay, I'll submit the patches with updates. After testing over 2000 iterations of pahole, I haven't seen a segmentation fault so I _think_ the locking in patch 2 is sufficient to avoid crashes. Thanks! Alan >> >> Alan >