mirror/zig
1
0
Fork 0
mirror of https://github.com/ziglang/zig.git synced 2026-02-21 16:54:52 +00:00
Code Issues Packages Projects Releases Wiki Activity

Cache: fix data race with is_problematic_timestamp

Browse Source 
Previously `recent_problematic_timestamp` was unprotected and accessed
potentially with multiple worker threads simultaneously.

This commit protects it with atomics and also introduces a flag to
prevent multiple timestamp checks from within the same call to hit().

Unfortunately the compiler-rt function __sync_val_compare_and_swap_16 is
not yet implemented, so I will have to take a different strategy in a
follow-up commit.
This commit is contained in:
Andrew Kelley 2021-12-09 21:14:39 -07:00
parent 4da83feccb
commit fdbb329d10
1 changed files with 26 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

34
src/Cache.zig
View File

@ -2,6 +2,7 @@ gpa: Allocator,
manifest_dir: fs.Dir,
hash: HashHelper = .{},
recent_problematic_timestamp: i128 = 0,
want_refresh_timestamp: bool = true,
const Cache = @This();
const std = @import("std");
@ -346,6 +347,12 @@ pub const Manifest = struct {
}
}
// Indicate that we want isProblematicTimestamp to perform a filesystem write in
// order to obtain a problematic timestamp for the next call. Calls after that
// in this same hit() function call will then use the same timestamp, to avoid
// writing multiple times to the filesystem.
@atomicStore(bool, &self.cache.want_refresh_timestamp, true, .Monotonic);
const file_contents = try self.manifest_file.?.reader().readAllAlloc(self.cache.gpa, manifest_file_size_max);
defer self.cache.gpa.free(file_contents);
@ -749,18 +756,29 @@ fn hashFile(file: fs.File, bin_digest: *[Hasher.mac_length]u8) !void {
fn isProblematicTimestamp(cache: *Cache, file_time: i128) bool {
// If the file_time is prior to the most recent problematic timestamp
// then we don't need to access the filesystem.
if (file_time < cache.recent_problematic_timestamp)
var ts = @atomicLoad(i128, &cache.recent_problematic_timestamp, .Monotonic);
if (file_time < ts)
return false;
var file = cache.manifest_dir.createFile("timestamp", .{
.read = true,
.truncate = true,
}) catch return true;
defer file.close();
if (@atomicRmw(bool, &cache.want_refresh_timestamp, .Xchg, false, .Monotonic)) {
var file = cache.manifest_dir.createFile("timestamp", .{
.read = true,
.truncate = true,
}) catch return true;
defer file.close();
cache.recent_problematic_timestamp = (file.stat() catch return true).mtime;
const new_ts = (file.stat() catch return true).mtime;
ts = if (@cmpxchgWeak(
i128,
&cache.recent_problematic_timestamp,
ts,
new_ts,
.Monotonic,
.Monotonic,
)) |race_ts| race_ts else new_ts;
}
return file_time >= cache.recent_problematic_timestamp;
return file_time >= ts;
}
// Create/Write a file, close it, then grab its stat.mtime timestamp.