Re: [RFC][gdb/symtab] Lazy expansion of full symbol table

[Tom de Vries <tdevries@suse.de>]

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On 6/18/21 4:30 AM, Tom Tromey wrote:
> Tom> I did an overnight build and test with the updated branch (5bc56d745fd)
> Tom> and ran into some trouble.  The first internal-error I investigated
> Tom> happens when parsing the libstdc++ .debug package (so, it was not
> Tom> specific to the test-case).  It seems the branch has some trouble with
> Tom> the dwz layout where an abbrev entry is shared between different CUs:
> 
> Thank you for trying this, it uncovered several bugs.
> As you can see I haven't gotten to the dwz testing yet... one of the
> issues with DWARF, btw, is that there are just so many modes.
> I.e., I haven't tried DWO or .debug_types yet either.
> 

Yeah, very true.

> I pushed some patches to fix the crashes but the result is so fast that
> I suspect it is incorrect:
> 
>     (gdb) file libstdc++.so.6.0.28-10.2.1+git583-lp152.4.1.x86_64.debug 
>     2021-06-17 20:25:34.361 - command started
>     Reading symbols from libstdc++.so.6.0.28-10.2.1+git583-lp152.4.1.x86_64.debug...
>     2021-06-17 20:25:34.406 - command finished
>     Command execution time: 0.075291 (cpu), 0.045521 (wall)
> 
> (Though /bin/gdb is also pretty fast here, maybe I'm doing something
> else wrong.)
> 
> So, at least it doesn't crash, but more investigation is needed.
> I'll probably add some code to make it easy to dump the index so it's
> easier to see what the scanner recorded.

Tried the updated branch and ran into a race condition, fixed in
attached patch.

Thanks,
- Tom

[-- Attachment #2: 0001-Fix-race-condition-in-dwarf2_section_info-read.patch --]
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Fix race condition in dwarf2_section_info::read

It can occur in dwarf2_section_info::read that:
- thread A sets readin to true, and buffer to nullptr, and then
- thread B reads readin as true, returns and finds that buffer is nullptr
  and concludes that there no such such section.  Which is not true, it's
  just that thread A hasn't finished reading it yet, which would make
  buffer != nullptr.

---
 gdb/dwarf2/section.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/gdb/dwarf2/section.c b/gdb/dwarf2/section.c
index 8e1c0197c85..e6940103d77 100644
--- a/gdb/dwarf2/section.c
+++ b/gdb/dwarf2/section.c
@@ -30,6 +30,8 @@
 #include "objfiles.h"
 #include "complaints.h"
 
+#include <mutex>
+
 void
 dwarf2_section_info::overflow_complaint () const
 {
@@ -118,10 +120,12 @@ dwarf2_section_info::empty () const
 void
 dwarf2_section_info::read (struct objfile *objfile)
 {
+  static std::mutex mutex;
   asection *sectp;
   bfd *abfd;
   gdb_byte *buf, *retbuf;
 
+  std::lock_guard<std::mutex> guard (mutex);
   if (readin)
     return;
   buffer = NULL;